1. Field of the Invention
The invention relates to the field of optics, and more specifically, to methods and structures for improving the manufacturability of Bragg gratings and systems that include Bragg gratings.
2. Related Art
Optical spectral filters are a critical component in modern optical systems. A Bragg grating is a type of optical spectral filter that is often used to filter and select specific wavelengths in optical systems. Conventional Bragg gratings are formed by a series of evenly spaced grating peaks that reflect a particular wavelength of light. Because of this fixed-pitch characteristic, conventional Bragg gratings can be referred to as “constant pitch” Bragg gratings.
FIG. 1A shows a conventional constant pitch Bragg grating 121A that includes a plurality of grating peaks 122A formed on a substrate 120A. Grating peaks 122A are all spaced equally from one another at a fixed pitch P_BRAGG. When pitch P_BRAGG satisfies the “Bragg condition” for a particular wavelength, it will strongly reflect that wavelength of light. The Bragg condition for the fundamental wavelength is defined by the following:P_BRAGG=λ/(2*neff)  (1)where λ is the wavelength of the reflected light and neff is the effective refractive index of the optical waveguide. Because of this relationship, pitch P_BRAGG can be referred to as the “Bragg pitch” for wavelength λ. Similarly, wavelength λ can be referred to as the “Bragg wavelength” for pitch P_BRAGG.
Distributed-feedback (DFB) lasers make use of this filtering characteristic of Bragg gratings to generate single wavelength lasers. FIG. 1B shows a conventional DFB laser 100 that could be produced using constant pitch Bragg grating 121 shown in FIG. 1A. DFB laser 100 includes an active region 110 formed between an upper boundary region 120 and a lower boundary region 130, which in turn is formed on a substrate 140. An upper contact is formed over upper boundary region 120, while a lower contact 150 over the bottom surface of substrate 140. Upper boundary region 120 includes constant pitch Bragg grating 121.
Upper boundary region 120, active region 110, and lower boundary region 130 form a laser diode structure that produces light emission within active region 110 when a current flows between upper contact 160 and lower contact 150. Cleaved ends 111 and 112 of the laser diode structure and constant pitch Bragg grating 121 form a resonating cavity that allows lasing to take place within active region 110, and a laser beam 190 is emitted, once sufficient current flows between upper contact 160 and lower contact 150. Without a grating the laser cavity will support a plurality of lasing wavelengths. The Bragg grating 121 performs a filtering function allowing the laser to lase at just a single wavelength, and thus discriminate between all of the other longitudinal modes of the cavity.
However, in a typical DFB laser, lasing can actually occur in one of two modes. FIG. 2A shows an example transmission curve T121 for constant pitch Bragg grating 121 shown in FIG. 1A. A stop band S1 (sometimes referred to as a “reject band”) bounded by wavelengths WL1 and WL2 indicates the region of low transmissivity for the Bragg grating.
Lasing can only occur at a transmission maximum within stop band S1. Specifically, lasing will occur at either wavelength WL1 or WL2, the two transmission maxima at either side of stop band S1. The actual lasing wavelength (mode), or the hopping characteristics between two modes, depends on many fabrication parameters such as waveguide width, material composition, and in particular, cleaving accuracy for the ends of the laser (e.g., cleaved ends 111 and 112 in FIG. 1B). Unfortunately, even state of the art cleaving technology is several generations away from being able to provide the cleaving accuracy that is required to produce a predictable lasing mode. Consequently, conventional DFB laser production is plagued by low yields, and by a manufacturing process where every single DFB laser needs to be tested to determine its lasing characteristics.
To overcome this bi-modal lasing problem, conventional Bragg gratings sometimes include a phase shift gap to adjust the transmission characteristics of a Bragg grating within its stop band. Specifically, the phase shift gap is intended to introduce an additional tranmission maximum in the center of the stop band. A phase shift gap is an enlarged spacing between two grating peaks in a Bragg grating that is designed to introduce a quarter wavelength shift between the between the grating peak groupings on either side of the phase shift gap (i.e., the light reflected by one of the grating peak groupings is dephased by 180° from the light reflected by the other grating peak grouping). Consequently, a Bragg grating that includes such a phase shift gap can be referred to as a quarter wavelength shifted (“λ/4-PS”) Bragg grating.
FIG. 2B shows a conventional A /4-PS constant pitch Bragg grating 221. λ/4-PS constant pitch Bragg grating 221 formed on a substrate 220 is substantially similar to constant pitch Bragg grating 121 shown in FIG. 1, except that Bragg grating 221 includes a central phase shift gap G_PHASE. Phase shift gap G_PHASE divides grating peaks 222 into two filter groupings 221-1 and 221-2. Within filter groupings 221-1 and 221-2, all grating peaks 222 are spaced by pitch value P_BRAGG. Phase shift gap G_PHASE is sized to introduce a quarter wavelength shift between filter groupings 221-1 and 221-2. This quarter wavelength shift requires that phase shift gap G_PHASE be half a period longer than the fixed pitch P_BRAGG (i.e., G_PHASE=1.5 times P BRAGG).
Note that λ/4-PS constant pitch Bragg grating 221 is still considered a constant pitch Bragg grating because phase shift gap G_PHASE does not adjust the Bragg wavelength of the Bragg grating. Filter groupings 221-1 and 221-2 are so named because they perform the actual “filtering” within Bragg grating 221. In other words, the constant pitch filter groupings 221-1 and 221-2 determine the position of the stop band for Bragg grating 221, just as the constant pitch of Bragg grating 121 shown in FIG. 1A determines the position of the stop band for Bragg grating 121. Phase shift gap G_PHASE does not change the position of that stop band. Rather, phase shift gap G_PHASE merely adjusts the behavior of the reflectivity curve within that stop band.
FIG. 2C shows an example transmission curve T221 for the λ/4-PS constant pitch Bragg grating 221 shown in FIG. 2B. Transmission curve T221 is superimposed on transmission curve T121 (dotted line) from FIG. 2A for comparative purposes. A stop band S2 (bounded by wavelengths WS1 and WS2) indicates the region of low transmissivity for transmission curve T221. Note that while stop band S2 is wider than stop band S1 shown in FIG. 2A, the two stop bands have the same location (e.g., both are centered around a wavelength WL3), because the constant pitch of filter groupings 221-1 and 221-2 are the same as the constant pitch of Bragg grating 121 (i.e., P_BRAGG). However, the introduction of phase shift gap G_PHASE into Bragg grating 221 causes transmission curve T221 to have a maximum within stop band S2 at wavelength WL3. Wavelength WL3 will therefore always be the dominant mode provided by the Bragg grating, and a DFB laser that incorporates λ/4-PS constant pitch Bragg grating 221 will not be affected by cleaved end characteristics, and will lase only at wavelength WL3.
Note also that the contrast (i.e., the difference between maximum and minimum tranmission) of the transmission curve T221 is significantly reduced relative to transmission curve T121. This is due to the introduction of phase shift gap G_PHASE. Additional phase shift gaps can further reduce the tranmission contrast which in turn can unacceptably reduce the wavelength discrimination of the device. Furthermore, incorporating multiple phase shift gaps into a conventional Bragg grating can significantly complicate the manufacturing process, since a tool that has been calibrated to produce the fixed Bragg pitch will likely require re-calibration or re-adjustment at each phase shift gap (note that even adding a single phase shift gap can significantly increases the manufacturing complexity for conventional Bragg gratings). Therefore, conventional λ/4-PS Bragg gratings included at most a single quarter wavelength phase shift gap (although some academic research has been done on up to three identical phase shift gaps).
Bragg gratings are typically manufactured in one of three ways: (1) interference lithography, (2) photolithography, or (3) e-beam lithography. In interference lithography, two coherent beams are used to create a standing wave that exposes a periodic pattern into a resist layer. In photolithography, the desired grating pattern is incorporated into a photomask, which is then used to expose a photosensitive layer. And in e-beam lithography, an electron beam is scanned directly across an electron-sensitive layer to create the desired pattern.
Each of these techniques is constrained by a minimum placement resolution (i.e., the accuracy with which a tool can address locations on the target piece) that limits the grating pitch values that can be produced. Typically, e-beam lithography is used to manufacture Bragg gratings, due to the generally better placement resolution characteristics of such tools. For example, the minimum placement resolution of a particular e-beam lithography tool could be 10 nm, in which case the grating pitch values produced by that tool would be limited to multiples of 10 nm. On the other hand, a different e-beam lithography tool might have a minimum placement resolution of 25 nm, in which case the grating pitch values produced by that tool would be limited to multiples of 25 nm.
This type of tool-specific limitation can make manufacturing a conventional Bragg grating problematic, since the desired Bragg pitch may not necessarily be compatible with the minimum placement resolution of a particular tool. For example, communication on the International Telecommunications Union (ITU) grid (ITU-T G.694.1 (2002)) at 195 THz requires a 1537.40 nm wavelength laser, which in turn requires a 240.144 nm constant pitch value (using equation (1) and assuming an effective refractive index neff of 3.201). Unfortunately, the typical minimum placement resolutions of modern lithography tools do not coincide with this pitch value.
For example, the LEICA™ EBPG-5 and HITACHI™ HL800D lithography tools have minimum placement resolutions of 5 nm and 10 nm, respectively. Therefore, those tools could write grating pitches that are multiples of their minimum placement resolution; i.e., 240 nm, 245 nm, or 250 nm pitch values would be achievable with the LEICA™ tool, while 240 nm and 250 nm pitch values would be achievable with the HITACHI™ tool. However, the 240.354 nm target pitch (which is not a multiple of 5 nm or 10 nm) would not be achievable with either tool, based on their standard minimum placement resolutions. This limitation can make DFB laser production for modern communication systems very difficult, since such systems typically require extremely fine wavelength control.
For instance, the channel adjacent to the 195 THz channel on the ITU grid is 194.9 THz (1538.19 nm). Using the same assumptions that were applied to calculate the Bragg grating pitch for the 195 THz channel, the 194.9 THz channel requires a 240.267 nm pitch. Thus, the difference between pitch values for the adjacent channels is 0.123 nm, which is far less than the minimum placement resolution of modern e-beam lithography tools.
One technique to overcome this minimum placement resolution problem is to apply a small scaling correction factor to the deflection system of an e-beam lithography tool. For example, a +0.06% correction to field magnification is sufficient to change the pitch value written by an e-beam lithography tool from 240 nm to the target 240.144 nm. State of the art e-beam lithography systems can have scaling accuracies better than 0.01%, making this approach a workable solution.
However, this type of scaling correction technique can be extremely difficult and time-consuming. A different scaling correction would be required before writing each desired pitch value, making single-pass writing operations for multiple Bragg gratings (having different fundamental wavelengths) impossible. Also, depending on the design of the e-beam lithography tool, additional calibration corrections would be required to minimize field and subfield stitching errors.
Another technique to overcome the minimum placement resolution of an e-beam lithography tool is to apply carefully controlled electron doses at selected locations that are compatible with the minimum placement resolution of the tool (“on-grid locations”). These electron doses are sized to create accumulated effective dose peaks at non-resolvable locations (“off-grid locations”). This technique, known as weighted-dose allocation variable-pitch e-beam lithography (WAVE) requires an accurate model of the forward and backward-scattered electron energy distributions in the resist and an algorithm to calculate the individual sub-dose values and locations. The WAVE technique can therefore be even more complicated than the scaling correction technique.
Accordingly, it is desirable to provide a technique for producing Bragg gratings that are compatible with standard lithography tool resolution requirements, but can also provide arbitrarily selectable wavelength filtering properties.